Delete deprecated & unstable range-specific step_by

[scottmcm]

Using the new one is annoying while this one exists, since the inherent method hides the one on iterator.

Tracking issue: #27741
Replacement: #41439
Deprecation: #42310 for 1.19
Fixes #41477

[rust-highfive]

r? @brson

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[brson]

r? @alexcrichton

[alexcrichton]

@bors: r+

[bors]

📌 Commit dcd332e has been approved by alexcrichton

[bors]

⌛ Testing commit dcd332e with merge 7a75d2b...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 7a75d2b to master...

[leonardo-m]

See also #43063 and #43064

[SimonSapin]

This is blocking Servo from upgrading Rust until we find a work around :/ Iterator:step_by is not a full replacement since it cannot support negative steps, and .rev() produces a different type. We have code that conditionally returns an iterator with positive or negative step.

[SimonSapin]

This was pointed out in #27741 (comment) when Range::step_by was deprecated.

[SimonSapin]

I ended up making this type. Would something like it belong in std?

struct MaybeReverse<I> {
    iter: I,
    reverse: bool,
}

impl<I: DoubleEndedIterator> Iterator for MaybeReverse<I> {
    type Item = I::Item;

    fn next(&mut self) -> Option<I::Item> {
        if self.reverse {
            self.iter.next_back()
        } else {
            self.iter.next()
        }
    }
}